Friday, August 29, 2014

Calculating the Required Network Bandwidth Transfer Of Redo In Data Guard Environments (Doc ID 736755.1)

Initially we had 1 RAC DataGuard in place, but after having 3 more RAC DataGuards (1+3), Our all Standby databases started lagging from Primary RAC Databases resulting into huge gap between them.

The reason was Network Bandwidth wasn't sufficient to handle this Redo Transport thru RFS over the existing network pipe. so I decided to use below formula to calculate required Network Bandwidth so that all 4 Dataguards shouldn't be faced any lag and should be in Sync with Primary.

The formula used (assuming a conservative TCP/IP network overhead of 30%) for calculating the network bandwidth is :

Required bandwidth = ((Redo rate bytes per sec. / 0.7) * 8) / 1,000,000 = bandwidth in Mbps

Measuring the Peak Redo Rate

Use the Oracle Statspack utility for an accurate measurement of the redo rate.

Based on your business you should have a good idea as to what your peak periods of normal business activity are. For example, you may be running an online store which historically sees the peak activity for 4 hours every Monday between 10:00 am - 2:00 pm. Or, you may be running a merchandising database which batch-loads a new catalog every Thursday for 2 hours between 1 am - 3 am. Note that we say "normal" business activity - this means that in certain days of the year you may witness much heavier business volume than usual, e.g. the 2-3 days before Mother's Day or Valentine's Day for an online florist business. Just for those days, perhaps you may allocate higher bandwidth than usual, and you may not consider those as "normal" business activity.However, if such periodic surges of traffic are regularly expected as part of your business operations, you must consider them in your redo rate calculation.

During the peak duration of your business, run a Statspack snapshot at periodic intervals. For example, you may run it three times during your peak hours, each time for a five-minute duration. The Statspack snapshot report will include a "Redo size" line under the "Load Profile" section near the beginning of the report. This line includes the "Per Second" and "Per Transaction" measurements for the redo size in bytes during the snapshot interval. Make a note of the "Per Second" value. Take the highest "Redo size" "Per Second" value of these three snapshots, and that is your peak redo generation rate.

Note that if your primary database is a RAC database, you must run the Statspack snapshot on every RAC instance. Then, for each Statspack snapshot, sum the "Redo Size Per Second" value of each instance, to obtain the net peak redo generation rate for the primary database. Remember that for
a RAC primary database, each node generates its own redo and independently sends that redo to the standby database - hence the reason to sum up the redo rates for each RAC node, to obtain the net peak redo rate for the database.

As an Alternative you can also get the 'Redo rate bytes per sec.' from V$SYSMETRIC_HISTORY, eg.
SQL> select * from v$sysmetric_history where metric_name = 'Redo Generated Per Sec';
or in a RDA-Output:
Performance - AWR Report - Statistic: "redo size"
Let us assume the redo rate is a 500 KB/sec.

Required bandwidth = ((Redo rate bytes per sec. / 0.7) * 8) / 1,000,000 = bandwidth in Mbps
Required bandwidth = ((512000/0.7) * 8) /1,000,000
Required bandwidth = 4.85 Mbps

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